Not applicable.
This invention is directed to the field of mass spectrometry and, more particularly, to sample probes with hydrophobic coatings for improved sequestration of a liquid sample to a probe feature.
Modern laser desorption/ionization mass spectrometry (xe2x80x9cLDI-MSxe2x80x9d) can be practiced in two main variations: matrix assisted laser desorption/ionization (xe2x80x9cMALDIxe2x80x9d) mass spectrometry and surface-enhanced laser desorption/ionization (xe2x80x9cSELDIxe2x80x9d). In MALDI, the analyte, which may contain biological molecules, is mixed with a solution containing a matrix, and a drop of the liquid is placed on the surface of a probe. The matrix solution then co-crystallizes with the biological molecules. The probe is inserted into the mass spectrometer. Laser energy is directed to the probe surface where it desorbs and ionizes the biological molecules without significantly fragmenting them. However, MALDI has limitations as an analytical tool. It does not provide means for fractionating the sample, and the matrix material can interfere with detection, especially for low molecular weight analytes. See, e.g., U.S. Pat. No. 5,118,937 (Hillenkamp et al.), and U.S. Pat. No. 5,045,694 (Beavis and Chait).
In SELDI, the probe surface is modified so that it is an active participant in the desorption process. In one variant, the surface is derivatized with affinity reagents that selectively bind the analyte. In another variant, the surface is derivatized with energy absorbing molecules that are not desorbed when struck with the laser. In another variant, the surface is derivatized with molecules that bind the analyte and that contain a photolytic bond that is broken upon application of the laser. In each of these methods, the derivatizing agent generally is localized to a specific location on the probe surface where the sample is applied. See, e.g., U.S. Pat. No. 5,719,060 (Hutchens and Yip) and WO 98/59361 (Hutchens and Yip).
The two methods can be combined by, for example, using a SELDI affinity surface to capture an analyte and adding matrix-containing liquid to the captured analyte to provide the energy absorbing material.
In the practice of mass spectrometry, localizing the sample on the probe surface provides advantages. Localization provides more concentrated sample at the point of laser application. In the affinity version of SELDI, localization can be important because it allows the affinity reagent to capture more of the analyte, thereby providing greater sensitivity of detection. However, liquid samples tend to spread out over the surface of the probe, thwarting localization. This especially creates problems when the probe is designed to hold multiple samples and the samples cannot be sequestered to specific locations.
There is a need for better means for sequestering a liquid sample to a location on a probe surface.
This invention provides a mass spectrometry probe capable of sequestering liquid samples to specific locations, or features, of the probe surface. The probes comprise a substrate having a surface and a film that coats the surface. In general, samples used in mass spectrometry are dissolved in aqueous solutions. Therefore, the film is selected to be more hydrophobic than the surface (lower surface tension).
These coatings provide several advantages compared with mechanical borders. First, they avoid electrical field perturbations that hamper mass resolving power and mass accuracy. Second, they avoid areas of possible sample pooling and preferential crystallization in regions other than the probed area. Third, they avoid the need for maintaining strict mechanical tolerances such as in the case of elevated sample ridges or depressed sample wells, which can result in poor molecular weight determination accuracy and precision. Fourth, they avoid, unlike elevated margins, an optical stop which limits the probed area.
One solution to the problem that is not as effective involves manually applying a hydrophobic circle to the probe surface. The circle can be applied using a PAP pen, available from Polysciences (Warrington, Pa., USA). The PAP pen includes a hydrophobic material in an organic solvent base contained in a stylus. The coating is applied by drawing an enclosed line with the stylus on the substrate surface. The material delivered by the PAP pen has a contact angle of about 90xc2x0.
In one aspect this invention provides a probe that is removably insertable into a gas phase ion detector (e.g., a mass spectrometer) comprising: a) a substrate having a surface adapted to present an analyte to an ionization source and b) a film that coats the surface, wherein the film: i) comprises at least one opening that exposes the surface, thereby defining a feature for applying a liquid comprising an analyte; ii) has a water contact angle of between 120xc2x0 and 180xc2x0; and iii) has less surface tension than the substrate surface, whereby a liquid applied to the feature is sequestered in the feature.
In another aspect this invention provides a system comprising: a gas phase ion detector comprising an inlet port; and a probe of this invention inserted into the inlet port.
In another aspect this invention provides a method of detecting an analyte comprising: a) placing the analyte on a feature of a surface of a probe of this invention; b) inserting the probe into an inlet port of a gas phase ion detector comprising: i) an ionization source that desorbs the analyte from the probe surface into a gas phase and ionizes the analyte; and ii) an ion detector in communication with the probe surface that detects desorbed ions; c) desorbing and ionizing the analyte with the ionization source; and d) detecting the ionized analyte with the ion detector.